The therapeutic technique of applying dosages of radiation to tumors within the human body has been practiced for some time. Usually, only a slight difference will exist in the radio sensitivities of a tumor and the surrounding healthy tissue, with the former being somewhat more sensitive than the latter. Consequently, it is an important object in conducting radiation therapy to avoid as much injury as possible to the healthy surrounding tissue while delivering a homogeneous and adequate dose to the tumor. This presents an extremely difficult problem since the tumor comprises an irregularly configured, three-dimensional shape which is often situated well into the body and thus completely surrounded by healthy tissue.
Early attempts to minimize lethal damage to the healty tissue surrounding the treated tumor involved simply directing a rectangularly-shaped beam of radiation onto the tumor and rotating the beam around the patient (and thus, around the tumor) so that the resulting volume of tissue subjected to radiation was cylindrical in shape and included the irregularly configured tumor portion therewithin. Of course, this early technique resulted in destruction of a considerable amount of normal healthy tissue surrounding and sometimes enveloped by the tumor.
A more recent approach to the problem has succeeded in confining the treated area to the volume of the tumor and is commonly known in the art as "comformation" radiotherapy. Heretofore, conformation radiotherapy has involved the use of apparatus which conforms the beam of radiation to the shape of the tumor as the latter is rotated relative to the radiation beam. One method of practicing conformation radiotherapy involves interposing a beam shaping radiation shield between the source of radiation and the tumor, and then shifting the shield in synchronization with the rotation of either the patient or the radiation beam in a manner to continuously alter the cross-sectional shape of the beam as different sides of the tumor are exposed to such beam. Various types of apparatus for producing the needed synchronized shielding which continuously alters the cross-sectional shape of the beam have been employed in the past as discussed by Shinji Takahashi in: Conformation Radiotherapy, Department of Radiology, Nagoya University School of Medicine, Japan, ACTA Radiologica Supplementum 242, 1965 (see Chapter III, Pages 49-66). Prior art synchronized shielding apparatus typically have employed a multiplicity of differently configured, gear-driven cam elements, often operating on corresponding, shiftable, radiation shielding segments forming a diaphragm surrounding the radiation beam source. Each of the cam elements correspond to a geometrical section of a particular tumor volume of a individual patient and must therefore be tailor made for treating a particular tumor. As the radiation beam rotates around the patient, the cam elements are sychronously driven to continuously change the diaphram in a manner to alter the cross-sectional shape of the radiation beam to conform to the profile of the tumor, as the various sides or "profiles" of such tumor are presented to the beam while the latter rotates. From the foregoing, it is clearly apparent that the prior art apparatus for practicing conformation radiotherapy was particularly complex with respect to the mechanisms that were employed, moreover, the radiotherapy method of treatment using the mentioned prior art apparatus was particularly time-consuming, and therefore inefficient, since numerous mechanisms were required to be assembled and disassembled in the course of treating different patients.
To further complicate the problems associated with conformation radiotherapy, it is necessary to devise a means of protecting radiosensitive normal tissues and organs such as the spinal cord, kidneys, ocular lens, and small intestine, which organs lie in the path of the radiation beam, between the tumor to be treated and the radiation source. The prior art method of protecting healthy organs lying in the radiation beam path involved the placement of radiation absorbing structures between the source of radiation and the patient which function to reduce or "hollow out" the dosage of radiation applied to the healthy organs. These radiation absorbing structures possessed geometrical dimensions corresponding in direct proportion to the healthy organ to be protected, and in some cases were rotated in synchronization with the rotation of the radiation source where the healthy organ was of irregular shape. These radiation absorbing protective structures, commonly known in the art as "central absorbers" due to the fact that they absorb a portion of the radiation lying within central areas of the radiation beam, were less than completely effective in protecting the healthy organ because the thickness of the prior art absorber was dictated by the cross-sectional thickness of the organ being protected. This thickness limitation is due to the fact that the prior art method of protecting healthy organs requires that the central absorber be rotated relative to the radiation beam in a manner to cause various sides of the central absorber to be presented to the radiation beam as the latter rotates around the patient. Because the thickness of the central absorber is dictated by the cross-sectional area of the organ to be protected and must be directly proportional to the latter, the thickness of the central absorber is insufficient to completely absorb the radiation impinging thereon, and consequently allows a portion of such radiation to pass therethrough and onto the organ which is intended to be protected. Since it is optimally desired to prevent any radiation whatsoever from being applied to the healty organ, the prior art method of using rotating central absorbers having cross section geometries corresponding to the organ to be protected, are less than completely effective in shielding such organ from undesired radiation treatment. Moreover, it is quite clear that the complexity of the apparatus required to practice the previous method is considerably increased by the need for mechanism to rotate one or more central absorbers in synchronization with the shiftable shielding segments which continually change the cross section configuration of the radiation beam to correspond to different profiles of the tumor, while the radiation source is rotated around the patient. In fact, due to the excessive complexity, and therefore cost, of prior art radiotherapy apparatus of the type described above, conformation type radiotherapy using the "hollowed-out" technique has thus far enjoyed only limited use.
From the foregoing, it is readily apparent that there is a clear need in the art for a novel method of conformation radiotherapy which is considerably more efficient than prior art techniques, and which employs apparatus which reduces equipment cost and operating expenses to a level which will permit wide scale use of the conformation radiotherapy treatment method.